The present invention relates in general to diesel engine fuel injectors and the return spring which comprises one component part of most fuel injector designs. More specifically, the present invention relates to the use of a return spring with a variable spring rate in order to vary the natural frequency of the spring and thereby reduce the chances for premature fatigue failure.
Typically, fuel injector designs of the past several years have included a coil spring component which functions as a return spring for the follower. Such springs were designed so as to last as long as the designed life of the injector so that spring failure would not be the cause of injector failure.
More recently, some diesel engines have been equipped with hydraulic engine brakes which employ a hydraulic circuit including pistons, fluid passageways, and a direct interface with the exhaust valves of the engine The design of an engine brake is intended to slow the engine by keeping the exhaust valves open (approximately 0.007 inches (0.178 mm)) during the compression stroke of the engine.
What has been learned is that fuel injectors, which are part of diesel engines equipped with engine brakes, are prematurely failing due to failed (broken) return springs. As this data has been generated and gathered, there have been attempts to solve the return spring failure problem (i.e., the noticeable increase in failure rate), which appears to be limited to those diesel engines which are equipped with hydraulic engine brakes. It is believed that all prior attempts at solving this spring failure problem have focused on increasing the durability of the spring, basically making a stronger spring with a greater design margin. In effect, these prior attempts focused on lowering the stress level seen within the spring, but these attempts for the most part have proven to be ineffective.
Further study of the spring failure problem by the present inventors showed that the fuel injector return spring was oscillating severely at its natural frequency when the hydraulic engine brake was applied. The amplitude of this oscillation was severe enough to increase the stress range of the spring and eventually cause a fatigue failure. The return spring was actually operating outside of its design limits during the braking event. The spring was excited at or above its natural frequency due to an interaction with the engine brake. During the engine brake cycle, the exhaust valve opening and closing events were transmitted directly through the engine brake hydraulic circuit to the injector by way of the injector push tube. The forces and vibrations which are transmitted are sufficient to excite the spring at or above its natural frequency.
The present invention solves the problem of return spring fatigue failure by replacing the traditional injector return spring with a spring having a variable rate. The variable rate is created by reducing or varying the pitch of the coils near the end(s) of the spring. If resonance occurs with one harmonic, the end coils will close up, thus changing the natural frequency of the spring and tending to throw it out of resonance.
While the use of a variable pitch spring (i.e., variable rate) is known for use in conjunction with valves for internal combustion engines, no use of this concept has ever been attempted for solving a premature failure problem of injector springs due to an engine brake hydraulic circuit. In order to control stresses due to resonant vibrations in valve springs, several methods have been tried over the years, including making the natural frequency of the spring higher, modifying the cam contour, and reducing or varying the pitch of the spring coils near the ends of the spring. The prior use of this last option has been limited to valves due to the specific problems and issues represented by valves. It is not an obvious next step to use this particular spring design technique in order to redesign a fuel injector.